PARIS, France (Reuters) -- Thousands of French police patrolled central Paris on Saturday to enforce a ban on large gatherings and prevent urban unrest reaching the heart of the capital, including targets such as the Eiffel Tower.Police said they had monitored calls for violence on Internet sites and that potential targets included the Champs Elysees and the Eiffel Tower.In a sign of concern after violence rose slightly on the 16th night of clashes, regional authorities for the first time declared a curfew for minors in France's second city of Lyon for Saturday and Sunday night.Rioting by youngsters angered by unemployment, racism and lack of opportunities has generally dropped in intensity since President Jacques Chirac's government announced emergency measures on Tuesday including curfews.But the number of cars set ablaze by rioters rose slightly again on Friday night, and two fire bombs were thrown at a mosque in southern France, damaging the entrance.Some 502 vehicles were set ablaze across France, compared to 463 the previous night, and unrest hit areas including Strasbourg, Marseille, Lyon and Lille. But there were fewer incidents of violence in the Paris suburbs, police said."We've gone back to an almost normal situation in Ile de France (greater Paris region)," national police service chief Michel Gaudin told reporters.But warning of violence in the capital, he said: "One can easily imagine the places where we must be very vigilant."The Paris ban on large gatherings went into force at 10 a.m. (0900 GMT) and was due to run until 8 a.m. (0700 GMT) on Sunday. The heart of the city has largely escaped the violence that has rocked impoverished neighborhoods just outside the city.Three thousand extra police were brought into the capital on Friday, the Armistice holiday marking the end of World War One, and riot police have been patrolling key areas, important buildings and suburban trains since then.Mosque attackedPolice detained 206 people during the night, during which youngsters attacked a primary school in Savigny-Le-Temple southeast of Paris and destroyed its creche.Two shops were destroyed in Rambouillet, southwest of Paris, and a person on a scooter threw two fire bombs at a mosque and in the southern town of Carpentras before fleeing.There was no major damage and no one was hurt but Chirac, Prime Minister Dominique de Villepin and the French Council of the Muslim Faith (CFSM) swiftly condemned the attack."We firmly ask the authorities in particular to protect our mosques, which seem to be becoming the target of violent demonstrations and provocations," the CFSM said.Police said it was not clear if it was a racist act or an act of provocation.The top government regional official for the Lyon region announced a curfew for minors on Saturday and Sunday nights from 10 p.m. (2100 GMT) until 6 a.m. (0500 GMT) in Lyon and 10 communities near the city. Violence in the area rose overnight.The unrest was triggered by the accidental deaths of two youths who were electrocuted on Oct. 27 as they hid in a power substation while apparently fleeing police just north of Paris. It later spread to other cities and towns across France.Chirac and the government have been heavily criticized over their handling of the rioting, involving white youths as well as French citizens of Arab and African origin.Copyright 2005 Reuters. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.